1. Technical Field
The present invention relates to power detectors used to control the output power of RF transmitters. More particularly, the present invention relates to a power detector having a wide dynamic range which is used as a transmit power detector in a feedback loop to control the output power of RF transmitters.
2. Description of the Related Art
In second generation cellular radio telephone systems, e.g., time division multiple access (TDMA) systems, speech is encoded by a base station or mobile station into a digital format for transmission to a mobile station or base station respectively. In a TDMA system, for example, only one carrier is required to permit "N" users to access the assigned bandwidth for the carrier on a time basis. In TDMA systems, a framing structure is used to transmit user data in frames having a predetermined duration (T seconds). Each frame is defined by a predetermined number of slots (N) which corresponds to the number of users. Thus, each user is assigned a slot in the frame to transmit data. Such framing structure permits each user to gain access to the carrier for approximately 1/N of the time and generally in an ordered sequence. If a user generates continuous data at a rate of "R" bits/sec., the data must be transmitted in a burst at a higher rate, e.g., NxR, during each frame transmission. Thus, the data is transmitted in such digital systems in short bursts for short periods of time.
To transmit such short bursts of data, the RF transmitters in the mobile station are switched "on" for approximately 0.6 ms and switched "off" for the remainder of the data frame, e.g., 4 ms. Typically, if the switching characteristics of the transmitter are not within predefined parameters, the shape of the amplitude envelope of the transmitted data may vary. In certain cellular radio systems, the power level and amplitude envelope of the transmitted signal are specified by regulatory agencies. Typically, the base station transfers information to the mobile station which instructs the mobile station to transmit data at a particular power level within a predefined tolerance level established by such regulatory agencies. The desired dynamic range of such a communication system is between about 5 milliwatts and 3200 milliwatts. Variations from the required power level and the required amplitude envelope at any point along the wide dynamic range will affect the integrity of the system with respect to such regulatory agencies. To avoid variations in the amplitude envelope, the shape of the amplitude envelope must be precisely controlled particularly at the leading and trailing edges of the burst, i.e., at the rise and fall ramps of the burst. Precise control of the amplitude envelope at the leading and trailing edges of the burst is necessary, especially at transmitter switching times of approximately 10 to 30 microseconds. To achieve such precisely controlled amplitude envelopes, power detectors have been utilized in the transmit power network of the transmitter to control the amplitude envelope of the output RF signal.
One currently used technique for controlling the amplitude envelope is a conventional diode detector scheme in a feedback loop. However, utilization of the diode detector in the feedback loop causes the controllability of the power level to be reduced as the transmitter power level is reduced. Utilizing the diode detector scheme, the amplitude envelope of the output RF signal is adequately controlled at high power levels. However, at low power levels the amplitude envelope of the output RF signal cannot be adequately controlled. As a result, the dynamic range of current second generation cellular networks is limited by the characteristics of the diode detector.
Therefore, a need exists for a power detector network which provides a precisely controlled amplitude envelope of the output RF signal of the transmitter over the wide dynamic range of cellular communication systems. That is, a need exists for a power detector network which precisely controls the amplitude envelope of the output RF signal at low output power levels as well as high output power levels.